In the oil, gas, petroleum and power industries, various conditions may occur that necessitate immediate shutdown or tripping of the operations, process or plant. In those industries a majority of the final control elements of a shut-off system are implemented with fast acting shut-off valves. In such industries a majority of the shut-off valves remain open while the process is in a safe and controlled state. Such valves are closed only upon a plant trip rising from an out-of-control process or during a normal maintenance outage.
In practice, the testing of emergency shut-off valves is normally done during shutdown of the process. However, there is a tendency for such valves to stick or freeze due to corrosion or other reasons, which lead to an unsafe condition. The problem is exacerbated by economic conditions which may lead to a reduction in the frequency of shut-offs. For example, in some operations, a process may run continuously for one or more years without shutting down the process for maintenance.
State of the art emergency shut-off systems which control the shut-off valves have a number of features to detect plant or process failures and typically include redundancies for added reliability. However, such systems may not provide for the testing of shut-off valves itself. Other than stroking the valve the problem is that full stroking or completely closing the valve causes an undesirable disruption in the process.
A U.S. Patent of Albuaijan U.S. Pat. No. 6,435,022 discloses a partial stroke testing system. That systems is designed for implementation on an emergency shut-off valve with a main solenoid with manual reset, main solenoid valve, quick exhaust valve and a pneumatic actuator connected to a source of pressurized air supply for opening and closing the emergency shut-off valve and wherein the shut-off valve is normally movable between a fully open and a fully closed position. The system of the aforementioned patent includes control means programmed into the plant emergency shutdown system controller for initiating a test and for enhancing the bleed rate from the pneumatic actuator in the event of an emergency trip signal. Test means for testing the emergency shut-off valve without fully closing the emergency shut-off valve is included in the system. The test means includes a second solenoid and a second solenoid valve for bleeding off pressurized air to thereby move the emergency shut-off valve from full opened position to partially closed position. The system also includes an isolation valve for isolating the test means for maintenance purpose.
A more recent U.S. Patent of DeBruyne U.S. Pat. No. 6,935,610 discloses a partial stroke valve test apparatus. The apparatus is operably inserted between an emergency shutdown valve and a valve actuator, and comprises a drive cam attached to a drive shaft interconnecting the valve and actuator and an engagement cam attached to an engagement shaft which can be rotated from an operational position to a testing position to bring the engagement cam into operable communication with the drive cam. A key is locked to the engagement shaft to rotate the engagement shaft, and can be removed only when the engagement shaft is in the operational position. A safety stop mechanism prevents the engagement shaft from inadvertently rotating out of the operational and testing positions. A spring-biased pin engages radially-spaced detent seats in the engagement shaft to provide a positive indication of the position of the engagement shaft.
Finally, a U.S. Pat. No. 8,074,512 of Al-Buaijan discloses a partial stroke testing system coupled with a fuel control valve. The system for testing an emergency shut-off valve includes a first emergency shut-off valve and a control for initiating a test on the first emergency shut-off valve. The system also includes a fluid actuator for opening and closing the first emergency shut-off valve. A subsystem is also provided for testing the first emergency shut-off valve without fully closing the shut-off valve in response to a signal from the control. In this system, a second solenoid valve bleeds off pressurized fluid to move the emergency shut-off valve from a fully opened to a partially closed position functions as a second emergency shut-off valve. A second emergency shut-off valve is provided in series with the first emergency shut-off valve and a bypass around the second emergency shut-off valve allows the second emergency shut-off valve to be tested by being fully closed without shutting down the process. The use of the two emergency shut-off valves in series wherein either valve can shut down the process raises the safety integrity level.
Recognizing that the emergency shut-off valves can be stroked partially as a safeguard against frozen or stuck or slowly closing valves has led to a need for a simple, secure and reliable system for testing such valves without adversely effecting production. This approach also improves the safety of the operation.
The present system is also applicable to slow acting valves.
In processes in the oil, gas, petroleum and power industries, fuel to power the heaters, boilers or the like is fed to the associated burners through a fuel control valve and a fuel shut-off valve. The fuel control valve regulates pressure flow to the burners whereas the shut-off valve is normally in a fully opened state. The shut-off valve closes to cutoff fuel supply in the event of an emergency. From the safety standpoint of view, the shut-off action of the shut-off valve is of utmost importance. The operability of the shut-off valve can be confirmed by means of the online valve partial stroke testing. However, any failure in the internals of the shut-off valve may be to a fuel leakage to the heater. Introduction of a solenoid valve in the pneumatic circuit of the fuel control valve driven by an emergency shut-off signal is conceived as a means for supplementing the fuel shut-off action and the redundancy to achieve higher safety standards.
In addition, the partial stroke testing system in accordance with the present invention will not interfere with a plant trip i.e., a full shutdown due to an emergency condition. Furthermore, if a partial stroke test is being conducted at the time of a plant trip, the partial stoke test will contribute to a more rapid closing of the emergency shut-off valve. Thus, the partial stroking design acts as a backup to the main trip mechanism.
Accordingly, such systems should reduce the costs of insurance or risk coverage. In essence, the system partially closes the emergency shut-off valve to a predetermined position to test and confirm its ability to function and to ensure its availability on demand in the event of a plant emergency.